The present invention is directed to polishing pads used for creating a smooth, ultra-flat surface on such items as glass, semiconductors, dielectric/metal composites, magnetic mass storage media and integrated circuits. More specifically, the present invention relates to introducing an organic or inorganic coating throughout a thermoplastic pad using a supercritical fluid, thereby creating more suitable polishing pads.
Chemical-mechanical polishing (CMP)is used extensively as a planarizing technique in the manufacture of VLSI integrated circuits. It has potential for planarizing a variety of materials in IC processing, but is used most widely for planarizing metallizied layers and interlevel dielectrics on semiconductor wafers, and for planarizing substrates for shallow trench isolation.
In trench isolation, for example, large areas of field oxide must be polished to produce a planar starting wafer. Integrated circuits that operate with low voltages, i.e., 5 volts or less, and with shallow junctions, can be isolated effectively with relatively shallow trenches, i.e., less than a micron. In shallow trench isolation (STI) technology, the trench is backfilled with oxide and the wafer is planarized using CMP. The result is a more planar structure than typically obtained using LOCOS, and the deeper trench (as compared with LOCOS) provides superior latch up immunity. Also, by comparison with LOCOS, STI substrates have a much reduced xe2x80x9cbirds"" beakxe2x80x9d effect and thus theoretically provide higher packing density for circuit elements on the chips. The drawbacks in STI technology to date relate mostly to the planarizing process. Achieving acceptable planarization across the full diameter of a wafer using traditional etching processes has been largely unsuccessful. By using CMP, where the wafer is polished using a mechanical polishing wheel and a slurry of chemical etchant, unwanted oxide material is removed with a high degree of planarity.
Similarly, integrated circuit fabrication on semiconductor wafers require the formation of precisely controlled apertures, such as contact openings or xe2x80x9cvias,xe2x80x9d that are subsequently filled and interconnected to create components and very large scale integration (VLSI) or ultra large scale integration (ULSI) circuits. Equally well known is that the patterns defining such openings are typically created by optical lithographic processes that require precise alignment with prior levels to accurately contact the active devices located in those prior levels. In multilevel metallization processes, each level in the multilevel structure contributes to irregular topography. In three or four level metal processes, the topography can be especially severe and complex. The expedient of planarizing the interlevel dielectric layers, as the process proceeds, is now favored in many state of the art IC processes. Planarity in the metal layers is a common objective, and is promoted by using plug interlevel connections. A preferred approach to plug formation is to blanket deposit a thick metal layer on the interlevel dielectric and into the interlevel windows, and then remove the excess using CMP. In a typical case, CMP is used for polishing an oxide, such as SiO2, Ta205, W205. It can also be used to polish nitrides such as SI3N4, TaN, TiN, and conductor materials used for interlevel plugs, such as W, Ti, TiN.
CMP generally consists of the controlled wearing of a rough surface to produce a smooth specular finished surface. This is commonly accomplished by rubbing a pad against the surface of the article, or workpiece, to be polished in a repetitive, regular motion while a slurry containing a suspension of fine particles is present at the interface between the polishing pad and the workpiece. Commonly employed pads are made from felted or woven natural fibers such as wool, urethane-impregnated felted polyester or various types of filled polyurethane plastic.
A CMP pad ideally is flat, uniform across its entire surface, resistant to the chemical nature of the slurry and have the right combination of stiffness and compressibility to minimize effects like dishing and erosion. In particular, there is a direct correlation between lowering Von Mises stress distributions in the pad and improving polishing pad removal rates and uniformity. In turn, Von Mises stresses may be reduced though the controlled production of pad materials of uniform constitution, as governed by the chemical-mechanical properties of the pad material.
CMP pad performance optimization has traditionally involved the empirical selection of materials and use of macro fabrication technologies. For example, a pad possessing preexisting desirable porosity or surface texture properties may be able to absorb particulate matter such as silica or other abrasive materials. Or, patterns of flow channels cut into the surface of polishing pads may improve slurry flow across the workpiece surface. The reduction in the contact surface area effected by patterning also provides higher contact pressures during polishing, further enhancing the polishing rate.
Alternatively, intrinsic microtextures may be introduced into pad surfaces by using composite or multilayer materials possessing favorable surface textures as byproduct of their method of manufacture. Favorable surface microtextures may also be present by virtue of bulk non-uniformities introduced during the manufacturing process. When cross-sectioned, abraded, or otherwise exposed, these bulk non-uniformities become favorable surface microtextures. Such inherent microtextures, present prior to use, may permit the absorption and transport of slurry particles, thereby providing enhanced polishing activity without the need to further add micro-or macrotextures.
There are, however, several deficiencies in polishing pad materials selected or produced according to the above-described empirical techniques. Pads made of layers of polymer material may have thermal insulating properties, and therefore unable conduct heat away from the polishing surface, resulting in undesirable heating during polishing. Numerous virgin homogenous sheets of polymers such as polyurethane, polycarbonate, nylon, polyureas, felt, or polyester, have poor inherent polishing ability, and hence not used as polishing pads. In certain instances, mechanical or chemical texturing may transform these materials, thereby rendering them useful in polishing. Such pad surfaces, however, may require periodic reconditioning.
Polyurethane based pads for example, currently in widespread use, are decomposed by the chemically aggressive processing slurries by virtue of the inherent chemical nature of urethane. This decomposition produces a surface modification in and of itself in the case of the polyurethane pads. Such pads require reconditioning, and thus reduce the productivity and increase the cost of the polishing process.
Yet another approach involves modifying the surface of CMP polishing pads materials to improve for example, the wetability of the pad surface, the adhesion of surface coatings, and the application performance of these materials. Plasma treatment of polishing pad materials, for example, is one means to functionalize and thereby modify polishing pad surfaces. However, the simple functionalization of pad surfaces by plasma treatment is known to be a temporary effect, with spontaneous loss of functionalization after one to two days. While some success in the preservation of functionalized pad surfaces has been obtained for some fluorinated polymeric surfaces, this has not been demonstrated for other polymeric surfaces, and in particular, thermoplastics. Such modified surfaces may thus require periodic pad refunctionalization, or pad replacement.
Accordingly, what is needed in the art is an improved process for modifying a semiconductor wafer plastic polishing pad, thereby providing a rapid rate of polishing, but reducing the need for pad reconditioning and refunctionalization following a period chemical/mechanical planarization.
To address the deficiencies of the prior art, the present invention, in one embodiment, provides a polymer comprising a plastic substrate having a grafted compound located substantially throughout the plastic.
In another embodiment, the present invention provides a method for preparing a polymer that includes providing a plastic substrate and exposing the plastic substrate to a precursor in a supercritical fluid to thereby produce a modified plastic.
The foregoing has outlined preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention.